"Normal" microbiota can be causative agents of serious infections in cancer patients and infection prevention or elimination is essential. The objective of this study is to evaluate the effectiveness of new antibiotics against these bacteria. Four new cephalosporin-like agents, LY 127935, HR 756, C7174/E, and T-1551 were effective inhibitors of P. aeruginosa. HR 756 was the most active against the enterobacteria. LY 127935 and T-1551 were most effective against enterobacteria that were resistant to cephalothin. The Enterobacteriaceae were susceptible to BAYK 4999 and thienamycin as was P. aeruginosa to a lesser degree. Cross-resistance between thienamycin and ticarcillin and mezlocillin was rare. Mezlocillin was still active against most genera. The use of mezlocillin in the patient population resulted in an increased incidence of mezlocillin-resistant strains. Mezlocillin resistance can be due to beta lactamase production or intrinsic resistance.